The following invention represents an improvement to conventional subharmonic imaging of ultrasound contrast agents. The method improves a number of characteristics including the signal-to-noise ratio, the penetration, the axial resolution, and the specificity (i.e., the sensitivity to contrast agent relative to that of tissue).
There are a number of imaging modes that have been used to image ultrasound contrast agents. Among these are the fundamental mode, the second harmonic mode, the harmonic pulse-inversion mode, the power harmonic Doppler modes, the loss-of-correlation modes, and the subharmonic mode. Each of these modes has its advantages and its disadvantages in terms of penetration, detail resolution, frame-rate, ease-of-use, and other important imaging concerns.
One of the most recent of these imaging modes, the subharmonic mode, offers potential benefit over the others in its particular combination of advantages. Most notably, subharmonic imaging has very good specificity in its sensitivity to contrast agents over its sensitivity to tissue. It is also a mode that is not dependent upon destruction of the contrast agent and is therefore a mode that can be used in continuous, as opposed to intermittent, imaging. It is also a single-pulse method and is therefore highly immune to tissue motion artifacts.
In ultrasound contrast agents, the dominant subharmonic distortion component occurs at half the frequency of the insonification (i.e., half the fundamental frequency). In a typical subharmonic imaging scenario, the system is set up to launch a pulse that occupies a frequency band centered at the fundamental frequency f.sub.0, and to receive a band of frequencies centered at f.sub.0 /2. Echo signals at the fundamental frequency f.sub.0 are rejected, typically by use of a conventional bandpass filter centered at the subharmonic frequency. Note that this imaging scenario is quite similar to second harmonic imaging, except that instead of receiving and filtering at 2*f.sub.0, we receive and filter at f.sub.0 /2.
In subharmonic imaging one of the chief concerns is that the subharmonic signal levels are, for typical B-mode imaging pulses, typically quite low. The subharmonic distortion mechanism is such that the subharmonic distortion component accumulates gradually over the duration of the pulse. If the pulse duration is short, then the subharmonic distortion will not accumulate to as large a level as would occur if the pulse duration were longer. In other words, the subharmonic response from wideband excitation is substantially less than that from narrowband excitation. In still other words, there is a tradeoff between axial resolution and signal level. For practical imaging signal levels, the pulse durations must typically be substantially (e.g., a factor of three) longer than typical B-mode pulse durations. As a consequence, the axial resolution is typically substantially worse than that of typical B-mode.